“”To argue with a man who has renounced the use and authority of reason, and whose philosophy consists in holding humanity in contempt, is like administering medicine to the dead, or endeavoring to convert an atheist by scripture.

Like many other basic words, its meaning can be obscure and sometimes difficult. To be "rational" is generally considered to mean employing logical consistency and deriving appropriate conclusions from acceptable assumptions. Naturally, "consistency" and "appropriate" and "acceptable" all vary widely from person to person, which is one of the major problems with claiming to be "rational". Everyone will think they're being rational because they are happy with their assumptions and their own consistency. However, there are some accepted methods in place to try and standardise, such as in rationalism or science.

Typically, dictionaries define rationality as: the quality or state of being rational; the possession of reason; agreeableness to reason; reasonableness; the exercise of reason.[2][3]

Two types of rationality are often contrasted with each other: Epistemic and instrumental rationality. The former represents rationality in terms of determining and holding true beliefs. The latter consists of being able to make decisions that most efficiently achieve one's goals, whether or not said goals may be considered rational.[4] This dichotomy is sometimes argued to be a false one by those who see instrumental rationality as a special case of epistemic rationality, or deny that intentions provide sufficient reason to take action toward their realization.[5]

"Bounded rationality" is a term coined by Herbert Simon used to refer to a conception of agents opposed to "rational actor" models in which agents act with optimal or perfect rationality. Bounded rationality asserts that agents will not act optimally due to cognitive constraints. Instead, they do what Simon referred to as "satisficing," a portmanteau of "satisfy" and "suffice." That is, they will seek a solution that is "good enough" for their own purposes.[6]

The concept of ecological rationality draws on bounded rationality and the strategy of satisficing. Ecological rationality posits that cognitive biases and heuristics are not inherently irrational but in certain conditions are preferable to a strict adherence to formal logic and probability theory. This is because the extra effort expended to consider all information in the framework of formal models of rationality does not give enough extra benefit over using a simple heuristic to make a decision. Thus, employing rough heuristics may be more "rational" than using what are normally considered "rational" methods.[7]

Emotion is often contrasted with rationality. However, research in neuroscience and cognitive science, most notably by Antonio Damasio, has demonstrated that emotion is inextricable from reason. Damasio investigated patients with damage to the ventromedial prefrontal cortex, which mediates activity between higher-order thought in the frontal lobe and emotional responses in the limbic system. Patients with this type of brain damage did not lose their ability to think logically but suffered from a lack of emotional affect, rendering them unable to make ordinary, everyday decisions.[8]

This might be conceived of as a case similar to the problem of Buridan's ass. In this thought experiment, an ass[9] is placed in a position equidistant from two stacks of hay. The ass will move toward the closest stack of hay, but remains stuck in its initial position because neither one is closer. If the ass picks one stack at random to avoid starving, is this decision rational? This question has invited much navel-gazing over the definition of rationality in philosophical arenas.[10]

Critical Rationalism can be seen as complementary to serendipity in scientific discovery. Yet cognitive science made rationality a subject even in the area of finding new theories, and laws, started by the computational models in many machine learning areas. These movements focus on any rational process, and see scientific discovery as the main challenge of any creative rational learning process, in science, and in the long run, in general, by modelling the whole brain logically, on a computer.[11]

A number of thinkers have investigated the notion of rationality, or what is considered "rational," as being intertwined with power relations. Max Weber and Michel Foucault, for example, focused on the use of instrumental rationality in the process of "rationalization," i.e., how bureaucratic power imposes "rational" methods on citizens for its own ends.[12]

Rationality should not be confused with rationalism. Rationality is used to describe a quality of discourse or behavior; rationalism is a philosophy recognizing reason as the prime (or only) "way of knowing."